1. Field of the Invention
The present invention relates to an optical scanning unit which is used for image exposure or the like of an electrophotographing apparatus, and an image forming apparatus using this optical scanning unit. More particularly, this invention relates to an optical scanning unit which performs optical scanning with light from a light source by using of an optical scanning mechanism to irradiate the light on a photosensitive body, and an image forming apparatus using this optical scanning unit.
2. Description of the Related Art
Image forming apparatuses, such as a copying machine, a printer and a facsimile, employ a latent image forming type recording apparatus like an electrophotographing apparatus, due to a recent demand for image recording on normal sheets of paper. According to this image forming principle, after a photosensitive drum as a latent image carrier is precharged, the photosensitive drum is exposed to a light image to have an electrostatic latent image formed thereon. This electrostatic latent image is developed by a developing unit so that a toner image is formed on the photosensitive drum. This toner image is then transferred onto a sheet of paper.
This image forming apparatus performs image exposure line by line. This is accomplished by an optical scanning unit which uses a semiconductor laser device or the like as a light source and causes an optical scanning mechanism to scan with the light from the light source to irradiate that light on a photosensitive drum.
It is desirable that the light path in the optical scanning unit is not altered by external force applied.
FIG. 1 is an explanatory diagram of prior art. In an optical scanning unit 9, light from a semiconductor laser device 920 is transformed into parallel light by a collimator lens 921 and is focused by a cylindrical lens 922 to be irradiated on a polygon mirror 923 that is rotating. After this light is optically scanned by the polygon mirror 923, it is focused by a focus lens 925 and is directed downward by a reflector 926. The downward-directed light is guided by a reflector 927 to a focus lens 928 from which the light is irradiated on a photosensitive drum 90. As the semiconductor laser device 920 is driven by an image signal to be written on the photosensitive drum 90, an image corresponding to the image signal is written on the photosensitive drum 90 by Optical scanning.
At this time, the beginning of optical scanning should by synchronized with the image signal. To detect the beginning of the optical scanning, a start detecting unit is provided to detect the end portion (leading) of the scan light from the focus lens 928. In this example, a reflector 929 is provided at the position corresponding to the end of the scan light, so that the leading portion of the scan light is guided by the reflector 929 to a start detector 930, which generates an optical scan start signal.
This optical scanning unit 9 is normally mounted on the frame of the image forming apparatus. According to the prior art, the optical scanning unit 9 is secured to the apparatus' frame by screws or the like. This method causes deformation in the optical scanning unit 9 by the external force applied when the optical scanning unit 9 is attached to the apparatus' from or external force due to the tilting of the apparatus, thereby altering the optical path. This makes the aforementioned start detection difficult. This problem has been overcome by, for example, a method of adjusting the position of the start detecting mechanism 929, 930 after the optical scanning unit 9 is mounted to the apparatus' frame.
Since the prior art should adjust the start detecting mechanism after the attachment of the optical scanning unit 9 to the apparatus, therefore, the adjustment cannot be accomplished by the optical scanning unit alone. In other words, it takes time and labor to adjust the optical scanning unit 9 mounted on the electrophotographing apparatus. Further, the alteration of the optical path also changes the optical path on the irradiated surface of the photosensitive drum. Therefore, an error which does not matter in an electrophotographing apparatus of a low resolution would matter in a high-resolution electrophotographing apparatus. For instance, the error would result in the fluctuation of the vertical print lines in such an electrophotographing apparatus.